1. Field of the Invention
The present invention relates in general to projection systems, and more particularly to a projection system that uses an additional light valve in series with the conventional color splitting-converging prism, to increase the contrast of a projected image.
2. Description of the Related Art
A typical color digital projector comprises a lamp, an illumination system, and a light engine that includes a color splitting-recombining prism (e.g. plumbicon prism). The optical function of the light engine is to split uniform illumination light into Red/Green/Blue (RGB) channels, relay each of the colors onto a light valve, such as a DMD (Digital Micromirror Device), and then re-combine all three channels into a single illumination light beam that is projected on a screen via a projection lens.
The DMD is an electromechanical device consisting of millions of microscopic mirrors that modulate light by independently flipping each mirror through a predetermined angle. Using three such DMDs on the color splitting-recombining prism, a white light cone from the lamp is separated into red, green, and blue channels. Each color is individually modulated by a respective DMD and then recombined by the prism.
The DMDs modulate the light by turning the mirrors on and off several times during a video frame. A frame is divided into approximately 20 to 60 bit planes of different duration, based on bit sequence and frame rate. During a given bit plane each pixel on the screen is controlled by a single bit and is either driven ‘ON’ or ‘OFF’ for the entire duration of the plane. The number, duration, and location of the “ON” times are adjusted with respect to the timing of the frame for controlling the light level. Combining the ‘ON’ times for a given pixel gives the pixel its proper intensity.
It is possible to add an additional light valve, such as a white DMD, in series with the conventional color (RGB) DMDs in order to improve the image contrast, without any of the compromises or artifacts that result from the use of a dynamic iris. The additional light valve enhances contrast by reducing the light incident on the color DMDs on a pixel-by-pixel basis such that different intensity levels turn the light on and off at different times. However, because a DMD modulates light by turning the mirrors ‘ON’ and ‘OFF’ over time in a series of bit planes, adding an additional light valve results in the intensity of a white pixel being, in most cases, different than the intensity of one of the R, G or B values for that pixel (i.e. the white and color DMDs are ‘ON’ and ‘OFF’ at unrelated times). This, in turn, results in unpredictable color and intensity variations and image artifacts.
Although the problem of unpredictable color and intensity variations may be overcome somewhat by using certain different technologies for the additional light valve (e.g. LCOS, LCD), other problems are introduced through the use of such different technologies. In particular, these technologies operate so as to dim all of the light (i.e. not just the off-state light), so that the input signal must be amplified in order to compensate and bring the image light output back to its original level. The amplification must spatially match the white image. However, exact spatial matching is not possible in the presence of any convergence error whatsoever. Although the latter problem may be solved by softening the edges of the white image and the gain function applied to the image, the effective gain in contrast is thereby reduced. Also, the white image cannot include any steep intensity slopes since these will be converted to image artifacts by any convergence error. LCOS and LCD technologies are also heat limited and do not function well in a high brightness projector.